1. Field of the Invention
The present invention relates to an apparatus and a method for measuring characteristics of optical pulses.
2. Description of the Related Art
When the characteristics of optical pulses are directly measured by use of an optical detector, especially when the pulse width of the optical pulses is on the order of pico-seconds or femto-seconds, values indicating the pulse width of the measured pulses become broader than the actual values due to the frequency characteristics of the entire measuring system, including the optical detector. Therefore, use of this method is limited to measuring characteristics of pulses whose pulse widths are larger than 10 pico-seconds. When streak cameras or optical-sampling oscilloscopes are used, the measuring limit is on the order of hundreds of femto-seconds.
Although the auto-correlation method using non-linear optical crystal can detect pulses having a pulse width as short as 0.1 femto-second, it cannot detect weak optical pulses because it utilizes the higher-order non-linear optical effect. Moreover, since the measurable pulse width depends on the length of the crystal and the measurable wavelength depends on the material of the crystal, different crystals must be used for different purposes.
As a pulse width measuring method with highly sensitivity, the heterodyne method has been proposed (Reference:Tomaru et al.: "Optical pulse auto-correlation method using the heterodyne method," 43rd Applied Physics Symposium 26a-A-2,1996). In this method, the measured pulse width becomes broader than the actual one, due to the secondary dispersion of the frequency modulator used in the measuring optical system. Moreover, since optical detection of the heterodyne component of the higher harmonics is required, the optical detector must have an operation band width as broad as 10 GHz.
However, in order to use the above-described conventional techniques to measure the pulse characteristics of optical pulses, a broadband type optical detector and light of a high intensity are required, and sufficient measurement sensitivity is not obtained.